1. Field of the Invention
The present invention relates to an optical film measuring device. More specifically, it relates to an optical film measuring device for measuring film properties such as a thickness and chromaticity of a thin film having transparency.
2. Description of the Prior Art
A film thickness inspecting device having a structure such as shown in FIG. 1 is known as a device for measuring a film thickness of a thin film which is formed on a substrate such as a glass substrate or a semiconductor substrate by using reflected light of light with which the substrate is irradiated in a process of manufacturing a flat panel display (FPD) such as a liquid crystal display (LCD) or a plasma display panel (PDP).
In this film thickness inspecting device 1, a movable stage 2 is provided for mounting thereon a measurement target and moving it in a 2-dimensional direction in a horizontal plane, above which movable stage 2 a microscope 3 is arranged. The microscope 3 has such a configuration that below a lens-barrel portion 4 containing part of a microscope optical system an objective lens 5 is provided, above the lens-barrel portion 4 an eyepiece lens 6 is provided, and on a side of the lens-barrel portion 4 a light source 7 is provided. Above the microscope 3, there are provided a spectroscope unit 8 comprised of a diffraction grating 9 and a photo-detector 10 and a camera 11. Further, this film thickness inspecting device 1 is equipped with a signal processing portion 12 for calculating by operations a film thickness of a measurement target and outputting it to a display 13 for displaying an image in a viewing field (observation area) of the microscope 3.
In such a manner, in this film thickness inspecting device 1, a measurement target is set on the movable stage 2 beforehand and a light emitted from the light source is made incident into the lens-barrel 4 from the side. The light incident into the lens-barrel 4 is reflected by a half-mirror (not shown) provided in the lens-barrel 4 and passes through the objective lens 5 so that the measurement target may be subject to coaxial down-emission lighting through the objective lens 5. The microscope 3 has, as a viewing field range, a film thickness measurement point along its optical axis and an observation area surrounding the point, so that an image in the observation area picked up by the camera 11 is displayed on the display 13. Light reflected by the measurement target at a film thickness measurement point centered in the observation area of the light with which the measurement target has been irradiated passes through a pinhole (not shown) in a reflection mirror provided at an image formation position in the microscope 3 and is split by the diffraction grating 9, so that a split-light spectrum generated by the diffraction grating 9 is received by the photo-detector 10. When a measurement switch on the film thickness inspecting device 1 is pressed to turn it on, the signal processing portion 12 measures a film thickness of the measurement target in accordance with a predetermined algorithm based on a received-light-intensity signal received from the photo-detector 10 and displays a measurement result on the display 13. (Such a film thickness inspecting device is disclosed, for example, in Japanese Examined Patent Publication No. No. 1995-3365)
FIG. 2 shows a TFT substrate 14 of an LCD device as one example of a measurement target, in which an image in the observation area of the film thickness inspecting device 1 is displayed. The TFT substrate 14 has been obtained by forming a thin film transistor (TFT) or a wiring line on a surface of a glass substrate and forming a poly-imide film on it. A region on the TFT substrate 14 in which a thin film transistor or a wiring is formed is hereinafter referred to as a light blocking region (black matrix) 15, and a region surrounded by the light blocking region 15 is pixel opening 16. In a case where such a TFT substrate 14 becomes a measurement target, what is to be measured by the film thickness inspecting device 1 is a film thickness in the pixel opening 16 of, for example, the poly-imide film.
However, such a film thickness inspecting device 1 as described above, which employs a light interference method in film thickness measurement, is cable of measuring a film thickness only in a region (at film thickness measurement point P at the center of the observation area) where a regularly reflected light near the optical axis can be received. Therefore, if the film thickness measurement point P of the film thickness inspecting device 1 is positioned on the light blocking region 15, a film thickness of a poly-imide film in the pixel opening 16 cannot correctly be measured.
Therefore, conventionally, a film thickness of a poly-imide is measured after moving the TFT substrate 14 on the movable stage 2 so that a point where film thickness measurement is desired (i.e., film thickness measurement point P in the pixel opening in the TFT substrate 14) may be positioned directly below the optical axis as observing an image through the objective lens 6 under the microscope 3 or as confirming the image displayed on the display 13.
Therefore, a conventional film thickness inspecting device 1 takes time to measure a film thickness and finds it difficult to inspect all of measurement targets by mounting them to a production line and so has been obliged to sample them in inspection.